1. Field of the Invention
This invention relates to the fabrication of integrated circuit devices and more particularly to a method of local oxidation using stress-releasing narrow silicon trenches before the growth of field oxide in the fabrication of integrated circuits.
2. Description of the Prior Art
Local oxidation of silicon is the conventional lateral isolation scheme. The conventional local oxidation process (LOCOS) is described in VLSI Technology, International Edition, by S. M. Sze, McGraw-Hill Book Company, NY, N.Y., c. 1988 by McGraw-Hill Book Co., pp. 473-474. Referring to FIG. 1, a layer of silicon nitride 3 is deposited over a pad oxide 2 overlying a silicon substrate 1. The pad oxide is a thin thermal oxide which allows better adhesion between the nitride and silicon and acts as a stress relaxation layer during field oxide formation. The nitride and oxide layers are etched to leave openings 4 exposing portions of the silicon substrate where the local oxidation will take place. A boron channel-stop layer 5 is ion implanted into the isolation regions. Referring now to FIG. 2A, the field oxide 6 is grown within the openings.
There are many disadvantages to the conventional LOCOS method. Bird's beak encroachment 7 is caused by the lateral oxidation of silicon along the pad oxide under the nitride layer during the high temperature and long time required by the oxidation process. After the silicon nitride and pad oxide layers are removed, as in FIG. 2B, a bird's neck 8 and bird's head 9 are formed because of the stress relaxation inside the field oxide edge. The white ribbon effect, in which there exists a narrow region of nonoxidized silicon, is caused by the diffusion of nitrogen-like material from the compressive-stressed nitride layer edge into the neighboring underlying tensile-stressed pad oxide layer. The oxide thickness tends to thin in narrow openings because of the difference of oxygenic gas flow into different LOCOS opening sizes and the variation of nitride edge-induced stress. The non-recessed surface and tremendous stress that exists around the active area are caused by the inherent volume expansion that is about 2.2 times for the reaction from silicon to silicon dioxide. Many crystalline defects are generated by the relaxation of these tremendous stresses around the bird's beak.
U.S. Pat. No. 5,229,315 to Jun et al shows a trench etch around openings then trench filling followed by oxidation of center polysilicon to complete the isolation scheme. U.S. Pat. No. 5,004,703 to Zdebel et al uses a trench as isolation with sub-resolution dimension. A thermal oxide is used as a trench etch mask. U.S. Pat. No. 5,096,848 to Kawamura teaches oxidation at the center of an opening, then a trench etch around followed by trench filling. All of these methods are complex and have nothing to do with stress relaxation.